This invention relates to a control arrangement for thermostatically controlling both heating and cooling apparatus by means of a single heat-cool thermostat. Usually, a three-wire or three-conductor cable is needed to couple a remotely located heat-cool thermostat to the heating and cooling equipment to properly control that equipment. One of the conductors is common for both heating and cooling, while the second conductor is used only for heating control and the third only for cooling control. The necessity of a three-wire connection is particularly disadvantageous when air-conditioning equipment is added to a building which already has an existing thermostat-controlled heating system connected to the thermostat by only a two-wire cable. Installation of the required three-wire cable in an existing building where all the walls, ceilings and floors are finished is, of course, a time consuming and an expensive task.
The present invention obviates the need for a three-wire cable between the thermostat and the heating and cooling apparatus by providing a control arrangement requiring only a two-wire connection. Hence, rewiring is not necesary and the existing two-wire cable can be used for both heating and cooling control when cooling equipment is added.
The control system of the present invention is also useful in new construction since, in many installations, it is far less expensive than the conventional three-wire control.